This invention relates to an exhaust gas recirculation control method for internal combustion engines, which is intended to reduce a noxious component in the exhaust gases (NOx) without degrading the driveability of the engine, in a high load operating condition of the engine during exhaust gas recirculation at low temperature.
Exhaust gas recirculation, i.e. so-called EGR, is widely employed in internal combustion engines wherein part of exhaust gases from the engine is returned to the intake passage of the engine so as to reduce nitrogen oxides (NOx), one of noxious gases emitted from the engine.
In general, EGR is not effected over all operating regions of the engine, but the operating regions are divided into regions where EGR should be carried out, and regions where EGR should be inhibited, in response to various operating parameters of the engine. For example, an exhaust gas recirculation control method is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 62-93480, in which the EGR is not effected when the engine is in a cold state before being completely warmed up, while the EGR is also inhibited when the engine is operating in a high load condition, such as when the engine rotational speed, the absolute pressure within the intake passage, or the throttle opening exceeds a respective predetermined value.
According to the known exhaust gas recirculation control method, the inhibition of EGR at low engine temperature is intended to promote warming-up of the engine to ensure the stability thereof and also to prevent degradation of the driveability thereof. Although the amount of fuel to be supplied to the engine is usually increased so as to enhance the combustion condition and hence the driveability when the engine temperature is low, EGR would spoil the driveability which could be enhanced by the increased fuel amount. The inhibition of EGR under the predetermined high load condition is intended to enhance the accelerability of the engine under high load condition.
However, in recent years, it has been strongly desired to reduce NOx, e.g. to half of the conventionally allowable amount, from the standpoint of maintaining good environmental quality, and accordingly recent laws and regulations prescribe more strict requirements for reduction of NOx. The requirements can be met to a considerable extent by expanding the engine operating region in which EGR is to be effected.
An effective way to this end is to expand the EGR-effecting operating region even to a low engine temperature region, that is, to effect EGR even when the engine temperature is low. However, there arises a problem in applying the above known method to a low temperature region. For example, if in the low engine temperature region the predetermined value of intake passage absolute pressure above which EGR should be inhibited is set to the same value as that in a higher engine temperature region, it will degrade the driveability of the engine in a high load operating condition at low temperature, making it difficult to ensure satisfactory accelerability. Thus, it is difficult to expand the EGR-effecting region to a low engine temperature region without degrading the driveability.